1. Field of the Invention
The present invention relates to a process for producing conjugated dienes, such as isoprene, from tertiary alkyl ethers, such as MTBE (methyl-tert-butyl ether), and an oxygen source, such as air, over two functionally distinct catalysts.
2. Discussion of Background and Material Information
An isoprene synthesis process is disclosed in U.S. Pat. No. 4,593,145, KURARAY CO., wherein isoprene is produced by passing MTBE and formaldehyde through dilute aqueous acid solution in a stirred tank reactor under pressure. Substances which can release water and formaldehyde may be added between each catalytic stream. Although the conversion of formaldehyde is very high, the conversion of MTBE is moderate, and delicate control of reactor operation is required.
U.S. Pat. No. 3,574,780, SUMITOMO CHEMICAL CO., discloses another process for the manufacture of isoprene. In this process, isoprene is produced by passing the mixture of MTBE and air over mixed oxide catalysts. The feed MTBE is cracked to isobutylene and methanol over the catalyst. The product methanol is oxidized to formaldehyde which reacts with isobutylene over the same catalyst to produce isoprene. The catalyst preparation technique is described in Example 6. The acidic bismuth nitrate solution and the basic mixed solution of ammonium tungstate and molybdate are quickly mixed to form a precipitate at pH 5. This precipitate is kneaded with a silica sol. The mixture is dried and then calcined at 500.degree. C. The calcined mixture is crushed to 12-16 mesh granules.
JP 70-39,681, SUMITOMO, is directed to the production of isoprene from MTBE and oxygen using a catalyst of oxides of molybdenum, vanadium, tungsten or uranium, all of which are known oxidation catalysts, in a one-stage process.
U.S. Pat. No. 3,621,079, SUMITOMO CHEMICAL CO., is also directed to a process for the production of isoprene which involves passing a mixed gas comprising isobutylene, methanol, and dimethyl ether, steam and air over a mixed oxide catalyst.
Commercially, formaldehyde is produced via vapor phase, air oxidation of methanol over iron oxide-molybdenum oxide or silver catalyst. When methanol is oxidized over iron oxide-molybdenum oxide catalyst at 300.degree. C.-400.degree. C. and at atmospheric pressure, the overall yield of formaldehyde which has been produced is as high as 88%-91% carbon oxides, DME and formic acid being typical by-products.
ALBANESI, G. and MOGGI, P., Chem. Ind. (Milan), Vol. 63, P. 572-4 (1981) disclose that the acidic oxides such as alumina, silica-alumina and molecular sieves tend to decompose formaldehyde to CO and H.sub.2.
Several Russian publications teach the oxidation of MTBE and methanol to isoprene in a one-stage process. The catalysts can be supported on a silica gel, but acidity of the catalysts are not discussed in the abstracts.
BOLSHAKOV DA, YABLONSKAY AI, CHAPLIN DN, SU-452,189 (i. 86.07.30 f.72.06.15 72SU-797,692) is directed to the production of isoprene by catalytic oxidation of tert butyl methyl ether or mixtures of methanol and isobutylene; this document disclose a method which involves catalytic oxidation of tert-butyl methyl ether or mixtures of methanol and isobutylene at increased temperatures and in presence of catalyst based on W, Mo or Bi oxides which also contain a mixture of rare earth element oxides, i.e., didymium a waste from uranium production. It is disclosed that tests show that the addition of didymium in oxidation of tert-butyl-methyl ether eliminates the need for its preparation from methanol and isobutylene, and increases the yield of isoprene, using recirculation of reaction products, e.g., isobutylene and methanol, to 66%. In the case of oxidation of the mixture of methanol and isobutylene, the reaction is conducted in the gaseous phase, at 280.degree. C. and at a molar ratio of oxygen and isobutylene of 0.32, in presence of catalyst containing oxides of Si, W and didymium, increasing the yield of isoprene to 18.0 wt. % based on starting material isobutylene. The disclosed advantages are increased yield and elimination of by-products, especially formaldehyde.
YABLONSKAY AI, BOLSHAKOV DA, MOROZOVA LA, SU-415,910 (i.86.07.30 f.72.06.15 72SU-797,694) is directed to a catalyst for isoprene production from tert-butyl-methyl ether; the catalyst is described as containing tungsten oxide, didymium and silica gel. To improve efficiency, silica gel is impregnated in aqueous (NH.sub.4).sub.2 WO.sub.4 solution, the mixture is dried, and calcined in air. The didymium consists of a mixture of La, Nd, Pr and Sm oxides. A disclosed advantage is improved activity and selectivity of catalyst. In an example, 14.85 g silica gel is impregnated in an aqueous solution containing 21 g (NH.sub.4).sub.2 WO.sub.4 and 50 ml H.sub.2 O, the water is evaporated, the residue is dried at 100.degree. C., impregnated with solution containing 0.15 g didymium in 50 ml 10% HCl, evaporated, dried and calcined for 6 hours at 500.degree. C. in air. The yield of isoprene, using the obtained catalyst, was 12.8 mol. %.
YABLONSKAY AI, BOLSHAKOV DA, MOROZOVA LA, SU-415,906 (i.86.07.30 f.72.06.15 72SU-797,690) is directed to a catalyst for isoprene production which contains molybdenum, bismuth, vanadium, and additionally didymium. It is disclosed that the activity and selectivity of the catalyst, used for production of isoprene by oxidation of tert-butyl methyl ether, were increased by the addition of 0.1-15 wt. % didymium to the mixture of W, Mo, Bi and V oxides. The didymium consists of (in wt. %): 45 La oxide, 38 Nd.sub.2 O.sub.3, 11 Pr.sub.6 O.sub.11, 4 Sm.sub.2 O.sub.8 and 2 residues. A disclosed advantage is that the selectivity of the proposed catalyst is 100%, and the yield of isoprene is increased to 65 mol. %. In an example, 14.85 g silica gel is impregnated with solution containing 21 g (NH.sub.4).sub.2 WO.sub.4 and 50 ml H.sub.2 O, excess of water is evaporated and the solid is then treated with solution containing 0.05 g didymium in 50 ml 20% HCl. Solvent is evaporated and the catalyst is dried at 120.degree. C. and calcined for 6 hours at 500.degree. C. in an air stream.
JP 60-042-338, NIPPON-ZEON, relates to the reaction of MTBE and methanol to make isoprene using homogeneous catalysts. The catalysts are acidic and contain transition metals, but there is no indication of an oxidation process. An isobutylene source, such as MTBE, and formaldehyde are reacted with an acid catalyst containing a group IB or IIB metal ion to produce isoprene. JP 60-042,338, NIPPON ZEON, also discloses that certain trivalent metals (Cr, Mo, W, Sb) may be added to reaction mixtures as corrosion inhibitors. While these metals can catalyze oxidation, there is no indication that oxidation is involved. Also, this is a one-stage process.
JP 57-130-928, NIPPON ZEON KK, the preparation of isoprene which involves reacting at least one isobutylene source selected from isobutylene, tert butanol and alkyl tert butyl ether with a formaldehyde source in the presence of water in a liquid phase using an acid catalyst. At least one species of metal ion selected from the elements belonging to Groups Ia, Ib, IIa and IIb is present. A disclosed advantage includes the suppression of by-production of geraniolene while heightening the selectivity of isoprene. In an example, a stainless steel autoclave was charged with 4 wt. pts. of 25% aqueous solution of formaldehyde, 10 wt. pts. of water, 14 wt. pts. of tert butanol, 1 wt. pt. of 85% phosphoric acid, and 0.2 wt. pt. of disodium hydrogenphosphate, followed by charging 7 wt. pts. of isobutylene. The autoclave was immersed in an oil bath heated at 160.degree. C., and was started shaking, and the reaction was allowed to proceed for 25 minutes. The autoclave was then cooled, after which carbon tetrachloride was added. The organic layer was subjected to extraction and subjected to analysis by means of gas-chromatography. The yield of isoprene was 79.5 mol. % relative to formaldehyde and the by-production of geraniolene was only in the yield of 0.8 mol. % relative to formaldehyde.